1. Field of the Invention
The present invention relates to a backlight module and a liquid crystal display device, and more particularly to a backlight module and a liquid crystal display device adopting a new type light guide plate for improving the light use efficiency.
2. Description of the Prior Art
A backlight module is one of the important parts of a liquid crystal device. The liquid crystal can not emit light by self, so the function of the backlight module is to provide a uniform and high luminance light source for a liquid crystal display panel (LCD Panel) of the liquid crystal device, thereby making the LCD Panel normally display images. Generally, the backlight module mainly includes a light source, a reflect sheet, a light guide plate, multiple optical films (such as a diffuse sheet, a prism sheet and so on), a plastic frame and so on. The backlight module may be divided into a side illumination backlight module and a direct illumination backlight module according to the position of the light source located in the backlight module. The beam generated by the light source of the side illumination backlight module first enters into the light guide plate from a light incidence surface, then is multiple reflected by inside surfaces of the light guide plate and comes out from a light-emitting surface, and finally arrives at the optical films.
Therefore, in the side illumination backlight module, the light guide plate is an important element, which can affect the light transmission efficiency in the backlight module and the liquid crystal display device. Please refer to FIG. 1, in a prior side illumination backlight module 9, a light incidence surface 91 of a light guide plate 90 is generally upright for receiving the beam provided by a light source 91. In the inside of the light guide plate 90, the beam provided by the light source 91 is transmitted forward in the way of total reflection. Then the beam entering into the inside of the light guide plate 90 can be dispersed by circular or square mesh points printed on a bottom surface 92 of the light guide plate 90. And finally the beam entering into the inside of the light guide plate 90 can be emitted from a light-emitting surface 93 of the light guide plate 90.
Obviously, after entering into the light guide plate 90, the beam is multiple reflected. By simulating, it can be found that in a 42 inches backlight module, the beam emitted from a three millimeters thickness light guide plate can be generally reflected about two hundred times by a top surface of the light guide plate. In a larger size backlight module, the reflection time is more. Therefore, when the beam passes through the light guide plate, there exist two main losses. One loss is that the light guide plate can absorb light by self, and the other loss is that the printing ink for forming the mesh points can absorb light. The data shows that the reflectivity of the mesh points is about 97 percent. Namely, the beam can lose 3 percent of the light energy per reflection on the mesh point. If the beam is reflected five times by the mesh points, it will lose 14.2 percent of the light energy.
It can be seen that the light transmission efficiency and the light use efficiency of the light guide plate are low because the light guide plate can absorb light by self and the mesh points of the light guide plate can absorb light in short-wave band. Moreover, the printing ink of the mesh points can not uniformly absorb light. For example, it can absorb blue-green light wave band more than other bands that will results in the color of the whole top surface of the light guide plate being uniform. Namely, more serious chromatism may be occurred on the top surface of the light guide plate. For example, the factual color temperature of one end of the light guide plate near the light source is higher than the theoretical color temperature thereof, and the factual color thereof is biased toward blue. But the factual color temperature of the other end of the light guide plate far from the light source is too low.
Hence, it is needed to provide a new backlight module and a new liquid crystal display device adopting a new type light guide plate to solve the problem existed in the prior art.